A method of charging an assembled battery, includes mechanically connecting two external terminals of an assembled battery and two connecting lines of a charging circuit, respectively, the assembled battery including a plurality of lithium ion battery cells connected in series, a lithium ion battery cell of the plurality of lithium ion battery cells including a positive electrode active material including a first lithium compound comprising iron, the two external terminals, and a first communicating unit. The method also includes electrically connecting the first communicating unit of the assembled battery to a second communicating unit of the charging circuit, charging the assembled battery through the two external terminals using a quasi-constant voltage charging procedure, and reducing a charging current toward a late stage during the charging of the assembled battery.